1. Field of the Invention
This invention relates to a track jump control circuit for use, e.g., in compact disc (CD) players and a speed detector and speed control motor to be used for this track jump control circuit.
2. Description of the Related Art
As is known well, a disc reproducing device for CD players, video disc players and the like has a so-called search function whereby a desired portion of the disc can be searched based on a signal of recorded data. This search function is performed by causing a pickup for reading the data signal recorded on the disc to move in the radial direction of the disc at high speeds, or to jump over the tracks.
A track jump system of this type includes those disclosed in Japanese Patent Unexamined Publications Nos. 89282/1987, 276133/1986, 152565/1984, and 171080/1984.
The track jump system disclosed in the latter two patent publications performs the track jump function by counting the track signal recorded on the disc.
However, this type of system may sometimes not allow track detection with shorter track jump times because the frequency of transversing or jumping over the tracks nears that of the recording frequency. Any defects or stains also hamper correct reading of track signals, thereby leading to errors in counting the number of tracks. Vibrations to the device itself, inducing an out-of-focus error, may also result in erroneous tracking counts.
Both Japanese Patent Unexamined Publications Nos. 89282/1987 and 276133/1986 disclose a track jump system based principally on the linear motor system.
However, restrictions on the installation position of the motor are the problem associated with this type of system. Use of the linear motor in the position in which gravitational acceleration is applied to the direction of movement of the motor entails unnecessary dissipation. Application of a balancing load is not a solution because it increases the load output, which is not economical. Thus, in order to overcome this problem, it is generally considered that a rotary motor is used to reduce the speed through gears. However, a greater reducing ratio decreases the pickup transfer speed, while a smaller reducing ratio does not allow smooth low-speed transfer.
The system with the rotary motor will be explained taking an example of a CD player.
A pickup head during data reading (playback) must move ca. 30 mm in about 60 min. Thus, reliable controllability in very low moving speeds including 30 mm/3600 sec.apprxeq.0.008 mm/sec is required. On the other hand, in responding to the needs for a quicker music selection, for example, the pickup head is moved a distance of 30 mm in 0.2 sec. This demands controllability in a very high moving speed of 30 mm/0.2 sec.apprxeq.150 mm/sec. The difference between the two speeds is as big as 1 to 18,000 in ratio.
The maximum speed of a motor in general is about 3,000 rpm. To cause the pickup head to travel a distance of 30 mm in 0.2 sec, a gear ratio that moves the pickup head 30 mm with ten revolutions (3000 rpm/60 sec.times.0.2 sec) must be selected. If, however, the gear ratio is left unchanged, the motor makes 10 revolutions in 60 min during data reading. This means that the motor speed is as low as 0.003 revolutions per sec (10 revolutions per hour/3600 sec) and that no ordinary motor can perform the control required.
Awkward transfer of the pickup head at low speeds not only impairs the quality of reading the information signals but, in some worst cases, leads to overshoots and track jumps in tracking operation as well. Thus, in order to maintain smooth low-speed tracking, a negative feedback is used to apply a signal proportional to the speed to the motor.
However, it is generally difficult to extract a signal proportional to the speed of motor in rotation. In order to obtain such a signal, a generator constituted by a magnet and a coil is used to take out a voltage by rectifying the voltage induced across the coil with a mechanical brush. However, the use of the mechanical brush imposes the problem of poor durability of the system.
Too high a speed of the motor limits the mechanical follow-up of the brush and thus requires that the contact pressure be increased. This aggravates, however, the problem of durability.
As described above, the conventional track jump control circuit had the problems not only of inaccurate track jump and speed control adjustment between high and low speeds but also of restricted pickup position and poor durability.